A subsea well is drilled in one manner by first drilling or jetting to a first depth, then installing an outer or low pressure wellhead housing at the sea floor, with a first string of casing or conductor pipe extending to the first depth. The operator drills to a second depth, then lands a high pressure or inner wellhead housing in the outer wellhead housing. A second string of casing is attached to the inner wellhead housing and extends into the well to the second depth. The operator connects a drilling riser to the inner wellhead housing and drills the well to a third depth, which may be the total depth in some cases. The operator lands a casing hanger attached to a third string of casing in the inner wellhead housing. The operator might drill deeper and install a second casing hanger.
Once at total depth, in one method, the operator disconnects the drilling riser and runs a tubing hanger and a string of tubing on a completion riser. The tubing hanger has a production passage and an annulus passage, both extending from the lower end to the upper end of the tubing hanger. The completion riser has one conduit that connects to the production passage and another conduit that connects to the annulus passage. After the tubing hanger has been landed, the operator can circulate between the interior of the tubing and the tubing annulus on its exterior by pumping down one conduit and returning up the other. After the well has been perforated and tested, the operator lands a production tree on the inner wellhead housing. The tree has a production bore and an annulus bore. The operator orients or rotates the tree so that its passages align with the passages in the tubing hanger prior to landing.
In another method, before running the tubing hanger, the operator runs a different type of tree, commonly referred to as a “horizontal” tree. The operator runs the tubing hanger through the drilling riser and lands it in the horizontal tree. The tubing hanger has a production fluid side outlet that registers with a side outlet in the tree when properly oriented by the operator. The horizontal tree has a tubing annulus passage that extends from the bore of the tree below the tubing hanger seal to the bore of the tree above the tubing hanger seal. The drilling riser normally has a blowout preventer (BOP) on its lower end and a choke and kill line extending alongside. By closing the BOP on the tubing hanger running string, the operator is able to achieve circulation between the tubing annulus and the production passage in the tubing. A dual completion riser is not required, as in the first method described above.
In a third method, as shown in U.S. Pat. No. 6,715,554, the operator installs a tubing spool on the inner wellhead housing. The tubing hanger lands and seals in the tubing spool. The tubing spool has a tubing annulus bypass passage with a lower end and an upper end joining the bore. The seal of the tubing hanger is located between the upper and lower ends of the bypass passage. This arrangement enables the operator to circulate through the tubing annulus with a drilling riser connected to the tubing spool in the same manner as with a horizontal tree. The tree lands on top of the tubing spool, and production fluid flows up the tubing spool to the tree.